Probably the same reason 99% of steam-era structures have been torn down:
They aren't needed anymore.
And, railroads have a strong incentive to tear down unneeded structures: property taxes.
If they leave it up, but empty and unused, they are still taxed on it. Railroads tear things down the second they are surplus for that reason.
And, there is no need for any Class-1 railroad to store 10 locomotives under a roof when they can store hundreds of them outside without a building to maintain.
CR had an exposure to property taxes and they went an a concerted effort to reduce taxes paid. Anything they could tear down they did to save money. The Trenton cutoff was singled lined by first removing rails on one side at points along the line and they just rip it out.
Beth was just caught in the mess, Easton round house went about the same time but it was in worse shape but still used for storage.